1. Field of the Invention
The invention relates to a yarn or thread that expands upon heating. The yarn is especially intended for the manufacture of flat structures that are suitable for use as bedding mats for supporting exhaust gas catalysts in motor vehicles. Such bedding mats are arranged along the periphery of the ceramic body carrying an exhaust gas catalyst, and seal the annular gap between the ceramic carrier body and a metallic catalyst housing. Such bedding mats are expected to permit safe and shock-free bedding of the ceramic carrier body and have to compensate for any change occurring in the width of the annular gap to be sealed when the metallic catalyst housing expands at the operating temperature determined by the combustion exhaust gases.
2. The Prior Art
A mechanically solidified bedding mat made of textured yarns is known from Patent WO 00/36284. The yarns are fixed under tensile stress in the flat structure by quilting seams. The quilting seams are produced with a sewing thread whose thermal stability is lower than the operating temperature of the bedding mat. The quilting seams are destroyed when the bedding mat is heated. Deformation energy on the textured yarn is thereby released and the volume of the bedding mat increases. The expansion property of the bedding mat results from laying the textured yarns under tensile stress as they are being processed further to produce a mat. This tensile stress is mechanically fixed, and the possibilities available for manufacturing and conditioning these mats using this technique are restricted.
The invention provides a yarn from which fabrics, mats, knitted structures and similar flat structures can be manufactured that expand upon heating, and which can be used, for example as bedding mats for bedding exhaust gas catalysts in motor vehicles. The increase in volume is to be independent of the way in which the yarn is tied into the flat structure. Conventional methods for textile processing are to be applicable according to the invention.
It is an object of the invention to provide a yarn that expands upon heating. The yarn comprises a plurality of voluminous, heat-resistant individual filaments, which are aligned in parallel and compacted to form a bundle, and a casing for the bundle made of a material which has a lower thermal stability than the individual filaments, and which decomposes upon heating. Voluminous individual filaments means yarns having a textile appearance, preferably textured yarns. Such yarns are voluminous and have high elasticity. It is essential to the invention that such yarns are aligned in a bundle in parallel with each other and not twisted with each other. They are compacted under radial pressure and/or tension to form a bundle with high packing density, and fixed by the encasement of the bundle. Fabrics, mats, knitted or similar flat structures can be produced from the yarn on conventional textile processing machines.
The thermal stability of the yarn casing is lower than the temperature at which the yarn is used. When the yarn is heated for the first time to a temperature at which it is used, which is higher than the thermal stability of the casing, the casing is destroyed, and the deformation energy of the textured individual filaments stored in the bundle is released. The released deformation energy causes an expansion of the yarn, and the volume of the flat structure made from the yarn increases accordingly.